How To Have Fine Control Over Rotating Objects
This rig is over complicated: it has an IK bone that follows an empty, which rotates following a curve (circle); the animation is controlled by the evaluation time parameter of the curve, which has a cycle modifier in the graph editor, that you can see if you select the curve (and can be animated by hand using the 0-100 values, but it's difficult to achieve a smooth transition between different speeds due to its cyclic mode.



I think it would be better to delete the circle, the IK bone and the rotating empty, then select the bone called "prop rotate.L" in the "Armature prop L", set it as "Euler XYZ", and animate its Y rotation. So you can control the speed of the rotators by a simple use of ease in and ease out of a bezier curve in the graph editor.